|Publication number||US6946852 B2|
|Application number||US 10/405,880|
|Publication date||Sep 20, 2005|
|Filing date||Apr 2, 2003|
|Priority date||Mar 14, 2003|
|Also published as||US20040178803, WO2004083813A2, WO2004083813A3|
|Publication number||10405880, 405880, US 6946852 B2, US 6946852B2, US-B2-6946852, US6946852 B2, US6946852B2|
|Inventors||Michael A. Centanni|
|Original Assignee||Steris Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (48), Non-Patent Citations (14), Referenced by (5), Classifications (17), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention is a Continuation-In-Part (CIP) of U.S. application Ser. No. 10/389,036, filed Mar. 14, 2003, entitled “Method and Apparatus for Measuring Chemical Concentration in a Fluid,” which is incorporated herein by reference in its entirety.
The present invention relates to determining chemical concentrations, and more particularly to a method and apparatus for measuring the concentration of a chemical component in a gas mixture.
The degree of polarity of a molecule is expressed in terms of a “dipole moment.” Molecules, such as water, that exhibit a separation of charge within the molecule, have non-zero dipole moments. If the separated charges are equal in magnitude but opposite in sign, the magnitude of the dipole moment is equal to the product of the value of one of the separated charges and the distance of separation between the charges. The dipole moment is a vector that points from the negatively charged side of the molecule to the positively charged side of the molecule. The dipole moment depends on three factors, namely, (1) polarity of the molecule, (2) the magnitude of the separated charge, and (3) the geometry of the molecule. It is known that different molecules will have different dipole moments. For instance, molecules of antimicrobial chemicals, such as ozone (O3), and hydrogen peroxide (H2O2), have different dipole moments than molecules of water (H2O).
The present invention uses differences in the dipole moments of different molecules as a means for determining the concentration of a chemical component in a gas mixture.
In accordance with the present invention, there is provided a chemical concentration detecting system for determining a concentration of a first chemical component in a gas mixture, comprising: (1) a capacitor having first and second plates exposed to the gas mixture, said gas mixture being a dielectric therebetween; and (2) processing means for determining a change in an electrical property of the capacitor, said change in the electrical property varying according to the concentration of the first chemical component in the gas mixture.
In accordance with another aspect of the present invention, there is provided a method for determining a concentration of a first chemical component in a gas mixture, comprising: (1) exposing a capacitor, having first and second parallel plates, to the gas mixture, said gas mixture comprising a dielectric therebetween; and (2) determining a change in an electrical property of the capacitor, said change in the electrical property varying according to the concentration of the first chemical component in the gas mixture.
An advantage of the present invention is the provision of a concentration measuring system that uses a gas mixture as a dielectric of a capacitor.
Another advantage of the present invention is the provision of a concentration measuring system that will measure the concentration of a wide variety of chemical components, including antimicrobial chemicals.
Still another advantage of the present invention is the provision of a concentration measuring system that provides an accurate measurement of the concentration of a chemical component in a gas mixture.
Yet another advantage of the present invention is the provision of a concentration measuring system that is simple and inexpensive to manufacture.
These and other advantages will become apparent from the following description of a preferred embodiment taken together with the accompanying drawings and the appended claims.
The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail in the specification and illustrated in the accompanying drawings which form a part hereof, and wherein:
Referring now to the drawings wherein the showings are for the purposes of illustrating a preferred embodiment of the invention only and not for purposes of limiting same,
Sensor circuit 20 uses a capacitor to sense concentration of a chemical component in a gas mixture inside a chamber 100, as will be described in detail below. In this regard, it should be appreciated that the dielectric constant of a capacitor is dependent on electronic “polarizability.” Polarization is the ability of molecules to form a dipole under an electric field or the ability of the electric field to line up or rotate an inherent dipole, such as water molecules.
It should be understood that the term “gas,” as used herein, includes (a) chemical components that are gases at room temperature, and (b) chemical components that are in a vapor phase due to vaporization of a liquid.
In a preferred embodiment, the gas mixture includes at least one chemical component that is a decontaminant or sterilant, such as an antimicrobial chemical. Antimicrobial chemicals are active chemicals for a decontamination or sterilization process. The gas mixture may also include at least one chemical component that is “base” chemical. Base chemicals act as a diluent for an antimicrobial chemical, or as a vehicle or carrier for an antimicrobial chemical. The gas mixture also typically includes air as a chemical component. It is contemplated by the inventor that the gas mixture may include chemical components not specifically identified herein, as well as chemical components unrelated to a decontamination or sterilization process, including chemicals having different dipole moments.
In a preferred embodiment, processing unit 50 may take the form of a microcomputer or microcontroller, including a memory 52 for data storage. Processing unit 50 may also transmit control signals for the operation of other system elements, such as control means for controlling the production of a gas (e.g., a vaporization system), and/or for controlling the introduction of a gas into chamber 100 (e.g., a control valve or blower). Output unit 60 provides information in an audible and/or visual form. Accordingly, output unit 60 may take the form of an audio speaker and/or visual display unit.
Referring now to
Capacitor Cx is directly exposed to a gas mixture. In this regard, capacitor Cx is located inside chamber 100, wherein the gas mixture fills the gap between the conducting plates of capacitor Cx, thereby acting as an insulator or “dielectric” of capacitor Cx. Sensor circuit 20 provides data indicative of a capacitance Cx, corresponding to a chemical concentration. In this regard, capacitance Cx will vary in accordance with changes in the concentration of chemical components in the gas mixture.
It should be appreciated that the gas mixture may not be the sole dielectric in the gap between the conducting plates of capacitor Cx. In this regard, it is contemplated that one or more solid dielectric materials may also be present in the gap, including, but not limited to, organic or inorganic materials.
In a preferred embodiment, capacitor Cx is a parallel plate capacitor. However, it should be appreciated that capacitor Cx could be constructed in a different form. For example, Cx could be a cylindrical or spherical capacitor. If a spherical capacitor is used as capacitor Cx, holes must be placed in the outer shell of the capacitor such that the gas mixture can enter and exit the capacitor.
Electronic potentiometer 40 functions in the same manner as a mechanical potentiometer. In this regard, electronic potentiometer 40 is a three terminal device. Between two of the terminals is a resistive element. The third terminal known as the “wiper” is connected to various points along the resistive element. The wiper is digitally controlled by processing unit 50 (see FIG. 1). The wiper divides the resistive element into two resistors RBC and RAC. Electronic potentiometer 40 may take the form of a digitally programmable potentiometer (DPP™) available from Catalyst Semiconductor, Inc. of Sunnyvale, Calif.
In a preferred embodiment, voltage source 22 provides an AC voltage signal, such as a sinusoidal or pulse waveform. Null detector 30 is a device for detecting a null condition (i.e., a short circuit), such as a galvanometer, a voltmeter, a frequency-selective amplifier, and the like.
Operation of sensor circuit 20 will now be described in detail. The elements of the bridge circuit are connected between junctions AC, BC, AD, and BD. Electronic potentiometer 40 is operated by processing unit 50 to vary the resistances RBC and RAC until the potential difference between junctions A and B (VAB) is zero. When this situation exists, the bridge is said to be balanced or is “nulled.” The following relationships then hold for voltages in the main branches:
VAC=VBC, and VAD=VBD,
where VAC is the voltage between junctions A and C, VBC is the voltage between junctions B and C, VAD is the voltage between junctions A and D, and VBD is the voltage between junctions B and D. Accordingly,
V AD /V AC =V BD /V BC
V AD =V BD/(V AC /V BC)
The capacitance of capacitor Cx is connected between junctions A and D with a known capacitance of capacitor C1 between junctions B and D. Electronic potentiometer 40, connected from junction A to junction C to junction B, is adjusted by processing unit 50 to vary the voltages VAC and VBC.
When a null is detected by null detector 30, current I1 flows from junction C to junction A to junction D, and a current I2 flows from junction C to junction B to junction D. The voltage VAC across junctions A to C, and the voltage VBC across junctions B to C are:
VAC=I1RAC and VBC=I2RBC.
The voltage across a capacitor with capacitance C, current I, and frequency ƒ is:
Therefore, the voltages VAD and VBD may be expressed as:
As discussed above, VAD=VBD/(VAC/VBC), VAC=I1RAC, and VBC=I2RBC. Therefore,
In view of the forgoing relationship, when a null condition is detected, the resistance values for RBC and RAC, along with the known capacitance value of capacitor C1, can be used to determine unknown value of capacitance for capacitor Cx.
Chemical concentration detecting system 10 utilizes differences in dipole moments of different molecules to determine the relative concentration of a chemical component in a gas mixture. In the event there is only one chemical component in the gas mixture that has a measurable dipole moment, the concentration of the chemical component is determined. As discussed above, the gas mixture fills the gap between the conducting plates of capacitor Cx, thereby acting as a dielectric of capacitor Cx. By configuring capacitor Cx as an element of a bridge circuit, a measure of resistance values RAC and RBC, when the bridge is balanced or nulled, can be used to determine the capacitance of capacitor Cx. The capacitance of capacitor Cx is indicative of the relative concentrations of chemical components in the gas mixture, since the permittivity of the respective dielectric is affected by the relative concentrations of the chemical components in the gas mixture.
It is well known that for a parallel plate capacitor C=(kε0)(A/d)=(ε)(A/d), where C is capacitance, k is the dielectric constant, ε0 is the permittivity of free space (8.85×10−12 F/m), ε is the permittivity (Farads/meter) of the capacitor dielectric, A is the area of the capacitor plates (m2), and d is the separation in meters between the capacitor plates. As ε increases, the capacitance C will increase. Where the capacitor is a parallel plate capacitor with circular plates of diameter D, C=(πD2ε)/(4d).
It will be appreciated that the dielectric constant k of the capacitor can be determined according to the following expression:
where the value of capacitance, C, is determined as discussed above. The dielectric constant of the capacitor can also be determined by determining the capacitance with the dielectric in place between the conducting plates (Cd), and then determine the capacitance without the dielectric in place (Co). The ratio of the two capacitances equals the dielectric constant,
The response of a capacitor is influenced by the characteristics (e.g., frequency) of the AC waveform applied thereto. In this regard, capacitive reactance (Xc) is a function of frequency. Capacitive reactance is the opposition offered to the flow of alternating current by pure capacitance, and is expressed in ohms (Xc=1/(2πfC)). Accordingly, frequency of the waveform generated by voltage source 22 influences the response of capacitors. Thus, the frequency selected for voltage source 22 should preferably be a frequency that will provide a generally linear response for capacitance as the concentration of a chemical component in the gas mixture is varied. This will facilitate the use of interpolation and extrapolation of capacitance values, as will be discussed further below. If a suitable linear response is not obtained, then an expanded set of data points should be stored in memory 52.
It should be appreciated that while one embodiment of the present invention includes a sensor circuit 20 in the form of a bridge circuit, other types of circuits and techniques (including other types of bridge circuits, and capacitance meters) known to those skilled in the art, may be suitably used to measure capacitance. For example,
The equations governing sensor circuit 20B are as follows:
V s =V r [C y/(C y +C s)], therefore,
C y =V s C s /[V r −V s].
It is recognized that in some cases, the capacitance of the capacitor exposed to the gas mixture located in chamber 100 may be in the range of femtoFarad capacitance to low picoFarad capacitance (e.g., 1 fF to 100 pF), and that changes in concentration of a chemical component in the gas mixture may only result in a change of capacitance in the range of low picoFarad capacitance or even femtoFarad capacitances. Accordingly, the sensor circuit used to measure capacitance may need to have high sensitivity to allow for measurement of small values of capacitance. One high sensitivity sensor circuit is the charge transfer sensor circuit described above. Other high sensitivity circuitry is provided by such devices as the PTL 110 capacitance transducer from Process Tomography Limited of Cheshire, United Kingdom. The PTL 110 measures small values of capacitance (up to 10 picoFarads) with a resolution of 1 femtoFarad. A 1616 Precision Capacitance Bridge from IET Labs, Inc. of Westbury, N.Y., allows for measurement of capacitances in the range from 10−7 pF to 10 μF. Tektronix produces the Tektronix 130 LC Meter that measures capacitance from 0.3 pF to 3 pF. It has also been acknowledged in the prior art literature that capacitance sensor circuits using modern operational amplifiers and analog-to-digital converters (ADCs) can easily obtain resolutions to 0.01 pF.
With reference to
For example, processing unit 50 may store values of the capacitance of capacitor Cx that are determined for a plurality of relative concentrations of chemical components in a gas mixture, wherein the gas mixture includes at least one chemical component that is a decontaminant or sterilant, and air (at or below atmospheric pressure). As the relative concentrations of the chemical components are varied, the corresponding capacitance of capacitor Cx is determined, and stored in memory 52. For instance, capacitance of capacitor Cx may be determined for various relative concentrations of the chemical components. The recorded data corresponds with a curve associating capacitance to concentration.
In order to verify the accuracy of the data obtained using sensor circuit 20, it may be advisable to use an analytical tool to accurately measure concentrations of chemical components in the gas mixture. In this manner, only verified data is stored in memory 52. The analytic tool is preferably selected by the concentration ranges, size of region defined by chamber 100, desired response time, and duration of measurement. One analytic tool for use with measuring concentrations of a chemical component (e.g., vaporized hydrogen peroxide) is Fourier Transform Infra Red (FTIR) spectroscopy, or a high quality near-infrared (NIR) spectroscopy.
After the set of data is stored in memory 52, measurement of the concentration of a chemical component in the gas mixture can commence. Capacitor Cx is exposed to a gas mixture that is being monitored. As indicated above, capacitor Cx may be located in an enclosed chamber 100 filled with a gas mixture. A determination of RAC and RBC when the bridge is nulled is then used to determine a value for the capacitance of capacitor Cx. As discussed above, Cx=C1(RBC/RAC). A linear relationship between concentration and capacitance allows one to normalize any measurement made so as to provide the absolute concentration of each chemical component in the gas mixture.
When determining the concentration of an unknown, once the capacitance is measured, the data stored in memory 52 is searched for the capacitance of capacitor Cx to obtain the corresponding relative concentrations. If the capacitance of capacitor Cx is not found in the pre-stored data, the stored data may be interpolated or extrapolated to obtain a concentration corresponding to the measure capacitance of capacitor Cx. As noted above, frequency of the waveform generated by voltage source 22 will influence the response of capacitors. Where the capacitance of capacitor Cx does not exhibit a suitable linear response, an expanded set of data points should be stored in memory 52, so that interpolation or extrapolation is unnecessary.
It should be appreciated that while a preferred embodiment of the present invention uses a measure of a capacitor's capacitance to determine relative concentrations, it is also contemplated that a measure of other electrical properties of a capacitor may be used to determine relative concentrations, including, but not limited to, voltage, charge, reactance, current, permittivity, and dielectric constant.
Based upon the determined relative concentrations, processing unit 50 may be programmed to output control signals for modifying relative concentrations. For instance, processing unit 50 may transmit control signals (see
As indicated above, the gas mixture inside chamber 100 may include chemical components that are decontaminants or sterilants, such as antimicrobial chemicals, and chemical components that are base chemicals (i.e., diluents for an antimicrobial chemical, or vehicles or carriers for an antimicrobial chemical).
Examples of antimicrobial chemicals include, but are not limited to, vaporized hydrogen peroxide, vaporized bleach, vaporized peracid, vaporized peracetic acid, ozone, ethylene oxide, halogen containing compounds, ammonia gas, other gaseous oxidants, and mixtures thereof. The halogens of the halogen containing compounds include, but are not limited to, chlorine and fluorine.
Examples of base chemicals include, but are not limited to, de-ionized water vapor, distilled water vapor, a vaporized alcohol (e.g., a tertiary alcohol), and mixtures thereof.
Some typical examples of sterilizing atmospheres that may be created and whose concentrations or relative concentrations can be measured include, but are not limited to: ozone; vaporized hydrogen peroxide and water vapor; ethylene oxide; vaporized hydrogen peroxide, water vapor and ozone; vaporized hydrogen peroxide, water vapor and ethylene oxide; ozone and ethylene oxide; and vaporized hydrogen peroxide, water vapor, ozone and ethylene oxide.
In an alternative embodiment of the present invention, two sensor circuits 20 are used. Capacitor Cx1 of the first sensor circuit 20 is exposed to a gas mixture including (1) a first chemical component that is an antimicrobial chemical (e.g., vaporized hydrogen peroxide), (2) a second chemical component that is a base chemical (e.g., water vapor), and (3) air. Capacitor Cx2 of the second sensor circuit 20 is exposed only to the second chemical component and the air. Processing unit 50 calculates the difference between the two measured capacitances Cx1 and Cx2 to determine the concentration of the first chemical component. In this regard, the difference in capacitances Cx1 and Cx2 will be attributable to the concentration of the first chemical component of the decontaminant.
Other modifications and alterations will occur to others upon their reading and understanding of the specification. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.
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|U.S. Classification||324/663, 324/666|
|International Classification||G01N33/00, G01N, G01R27/08, G01N33/497, G01R27/26, C02F1/76, G01N27/22, G01N27/06|
|Cooperative Classification||G01N33/497, G01N27/228, G01N27/221|
|European Classification||G01N33/497, G01N27/22, G01N33/00D2, G01N27/22B|
|Apr 2, 2003||AS||Assignment|
Owner name: STERIS INC., CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:CENTANNI, MICHAEL A.;REEL/FRAME:013940/0087
Effective date: 20030402
|May 1, 2007||AS||Assignment|
Owner name: STERIS CORPORATION, OHIO
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:STERIS INC.;REEL/FRAME:019224/0624
Effective date: 20070401
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